The extracellular collagenous domains of the trimeric type I (SRI) and type II (SRII) macrophage scavenger receptors bind tightly to a surprisingly wide variety of polyanionic ligands, including modified proteins and lipoproteins, four stranded oligonucleotides, polysaccharides, endotoxins, and asbestos. These receptors may participate in atherosclerosis (foam cell formation), pathogen recognition, prevention of septic shock, asbestosis, and recognition and clearance of damaged proteins and cells. All of the proposed functions are directly related to health and disease, and they all stem from the broad binding specificity of these unusual endocytic receptors. In the work proposed here, we will determine the biochemical and structural bases for the broad binding specificity of type I and type II macrophage scavenger receptors and their mammalian and invertebrate homologues and analogues. We will identify key residues responsible for the receptors' distinctive binding properties (broad specificity, high and low affinity binding, complex ligand cross-competition, species- dependent binding differences, pH and thermal sensitivity, etc.). To do this, we propose a series of site-specific mutagenesis, ligand binding, and structural (e.g., electron microscopy) experiments. The predicted fibrous alpha-helical coiled coil and collagenous domains of these receptors are particularly well suited targets for mutagenic (e.g., alanine scanning and domain swapping) approaches to structure/function analysis. To determine if collagenous binding domains are used by other vertebrate and invertebrate scavenger-like receptors to generate their characteristic broad specificity, or if other structures can confer the same specificity, we will use expression and hybridization methods to clone other scavenger receptor genes. These include a novel mammalian scavenger receptor gene (SRIII),a macrophage SR gene from Drosophila melanogaster,and a putative homologue from C. elegans. The structures, binding properties and tissue distributions of these receptors will be compared to those of the mammalian SRI and SRII molecules. The goals of this work are 1) to define how a wide array of structurally diverse ligands bind to macrophage scavenger receptors, with special emphasis on a) the receptors' collagenous ligand-binding domains and b) comparison to other scavenger receptors, and 2) to provide both experimental tools and a biochemical framework with which to assess the functions of these unusual cell surface receptors. Detailed characterization of the structures and distinctive binding properties of scavenger receptors will provide important tools for the analysis of scavenger receptor function and will probably suggest new approaches for the treatment and prevention of at least some of the related diseases. For example, the proposed studies may lead to methods for predicting those physiologically relevant molecules which are receptor ligands; this would provide additional avenues for exploring receptor function, including the design of pharmacologic reagents.